1. Field of the Invention
This invention relates to magnetic clutch constructions, and more particularly it is concerned with an energizing device for an electromagnetic clutch suitable for use with a refrigerator for vehicles.
2. Description of the Prior Art
In one type of energizing device for an electromagnetic clutch known in the art, the energizing device comprises a main yoke member, a yoke assembly composed of an auxiliary yoke member of major diameter and an auxiliary yoke member of minor diameter, and an energizing winding mounted through an insulator in a space defined between the main yoke member and the auxiliary yoke member of major diameter.
The main yoke member which is substantially in the form of a cup includes a cylindrical portion and a bottom formed of a steel sheet by means of a press. The auxiliary yoke member of major diameter is cylindrical in shape and joined to the bottom of the main yoke member by force-fitting or welding. The auxiliary yoke member of minor diameter is also cylindrical in shape and joined to the bottom surface of the main yoke member by friction welding.
In the energizing device of the type described, three independent members are joined together into a single device which has at least two joints, so that it has hitherto been necessary to perform a time-consuming operation of forming the joints.
When the energizing device of the type comprising the aforesaid assembly of yoke members and energizing winding is assembled with components of an electromagnetic clutch, the surfaces of the cylindrical yoke members are arranged in juxtaposed relation to a rotary shaft with a very small clearance therebetween. Thus, in this type of energizing device, it has been necessary that its yoke members be maintained in concentric relation with one another. However, the plurality of surfaces of the device that should be maintained concentric with one another belong to different members, so that it is necessary to use one of such surfaces as a reference surface and to impart machining finishes to the rest of the surfaces. This would make the machining operation time consuming and troublesome.